Most aircraft of complexity include main propulsion engines which are employed in flight, as well as in taxiing, to generate not only energy for propulsion, but energy for use in running aircraft systems. Typically, energy is delivered to various systems in the form of so-called "bleed air", hydraulic fluid under pressure, and electrical energy. As is well-known, bleed air is employed in the operation of the aircraft's environmental control system which is to say air-conditioning system. Because the bleed air is typically taken from the compressor section of a gas turbine engine, it typically is at an elevated temperature and thus is used for de-icing purposes as well. In those instances where one engine may not be running and it is desired to start the same, bleed air may be also utilized to power an air turbine starter motor for the quiescent engine.
Hydraulic fluid under pressure is utilized to alter aircraft control surfaces, assist steering while on the ground, and elevate and lower the landing gear. Electrical energy is used for lighting, and powering various control systems on the aircraft as well as aircraft avionics.
In many instances, it is desirable to have one or more forms of energy available without operating the main propulsion engines. This is due to the fact that the main propulsion engines, to provide bleed air, hydraulic or electrical power without providing propulsion for the aircraft consume unnecessarily large quantities of fuel in the process. Consequently, to minimize fuel consumption and yet provide the necessary forms of power, so-called auxiliary power units or "APUs" have been deployed on many aircraft. In the usual case, an APU is a small gas turbine engine. The same is not designed for thrust production. Rather, it is designed so that the energy of burnt fuel will be converted into rotary motion of a turbine rotor which in turn is coupled through a transmission to electrical generators and hydraulic pumps to provide electrical and hydraulic power. And because the APU is a gas turbine engine, it necessarily will include a compressor for compressing air to be delivered to the APU combustor to support the combustion of fuel therein to drive a turbine wheel. Bleed air is thus available from the compressor of the APU or from a so-called "load compressor" driven by the APU.
While these units work well for their intended purpose, they are not as fuel efficient as they might be. In particular, demands for hydraulic power, electric power and bleed air may be and frequently are totally independent of one another. At the same time, as is well-known, gas turbine engines of the type employed in APUs operate most efficiently when operated at a constant speed. Consequently, when there may be a high demand for one or two forms of energy, say, electrical and hydraulic power, there may be a low demand for bleed air and in conventional practice, in such a case, much more bleed air than is required will be generated. Not infrequently, the system will dump or spill the excess bleed air which, of course, represents a waste of energy.
Furthermore, those APUs employing load compressors that are separate from the gas turbine compressor are more complex than is desirable as well as more expensive to manufacture.
As a result, attempts have been made to develop a dual or "split" flow centrifugal compressor. See C. J. Paine, "Dual Pressure Ratio Compressor" ASME 89-GT121 and G. Eisenlohr, "Stromungsprobleme bei einem Entnahmeverdichter neuartiger Konstruktion" VDI Berichte, Nr. 264, 1976. The objective was to provide, from a single stage centrifugal compressor, relatively low pressure bleed air flow to bleed air system and relatively higher pressure compressed air flow to the turbine combustor. Unfortunately, these proposals while demonstrating the feasibility of such systems, failed to achieve sufficiently high performance so as to be readily accepted technology in the field of APUs.
The present invention is directed to overcoming one or more of the above problems.